The September 11, 2001 Terrorist Attack (colloquially referred to as Nine-eleven and 9/11 for the US date format) was the deadliest act of terrorism in the United States, and among the deadliest single events of asymmetric warfare in history.
On the morning of September 11, 2001, four passenger jets were hijacked virtually simultaneously over the United States. Two were crashed into the World Trade Center in New York City, and one was crashed into the Pentagon (U.S. military headquarters) in Washington, D.C.. Both towers of the World Trade Center subsequently collapsed, and part of the Pentagon was destroyed in the ensuing fire.
The fourth hijacked plane crashed in a Pennsylvania field after passengers and crew tried to retake control of the plane from hijackers.

Casualties were in the thousands: 265 on the planes; 2650 people, including 343 firefighters who had rushed in, at the World Trade Center; and 125 at the Pentagon.

The WTC Towers were constructed almost entirely from steel coated in asbestos, and the attacks released dense clouds of dust into the air of Manhattan. Death usually follows asbestos inhalation only after fifteen years or more - so is possible that other victims of the attacks may die over the decades to come. However, studies in 2002 reported that the dust probably consisted of sufficiently large particles that the lifetime health risk was small compared to background risks.

Some passengers on the doomed flights were able to make phone calls reporting on events on board.
They reported that there was more than one hijacker on each plane (a total of 19 were later identified) and that they took control of the planes using box-cutter knives.

The terrorist attacks of September 11th 2001 had immediate and overwhelming effects upon the United States population and resulted in a global outpouring of memorials and services as well as tolerance of the US retaliation upon those accused of supporting the attacks. Political effects included legal and budget reforms to enable government agencies to arrest foreign nationals in other countries or members of the public who might incite subversive thoughts.
There was also a significant amount of celebration in some communities hostile to US policies, which Americans generally found puzzling, disturbing and highly offensive. The massive undertaking of rescue and recovery took months, with the clean up continuing through May. The task of providing assistance to the survivors and the families of victims is ongoing.

Though no group has claimed responsibility, the U.S. government immediately launched a full-scale response, stating its intentions to go to war against those responsible.
In late September, British Prime Minister Tony Blair released evidence compiled by Western intelligence agencies connecting Osama bin Laden, a wealthy Saudi terrorist with close ties with the Afghan Taliban leadership, and bin Laden's Al Qaida organisation.
The Taliban refused to extradite Osama bin Laden and all other al Qaida leaders based in Afghanistan without a proof of guilt. That proof was not given and a coalition led by the United States launched an attack in Afghanistan on October 7.
After the U.S. attack removed the Taliban from power, a videotape was discovered abandoned in Kabul, the Afghan capital, which showed bin Laden discussing the attacks in a context which makes clear that he had foreknowledge.

Following the attack, the United States government has been on heightened alert for new terrorist attacks, periodically warning of "imminent attacks".
In late September, cases of anthrax started breaking out; no connection to the September 11 attack has been found, though reports emerged in March 2002 that one of the hijackers was infected with cutaneous anthrax.

The fires at the World Trade Center site continued to burn for three months while rescue workers removed and sifted through debris. Much debris was sent to the National Institute of Standards and Technology for analysis, including one steel beam known to have been struck by an airliner (from building codes).
Five months after the attack, the last survivors were released from the hospital.
By six months afterwards, the 1.5 million tons of debris had been removed from the WTC site and work continued below ground level despite concerns that the seawall around the site might collapse.
Ceremonies marking the end of the debris removal took place at the end of May 2002.

Historical Note on the term Nine-eleven (or 9/11 in the US date notation for September 11th.) Some people refer to the September 11, 2001 Terrorist Attack date as Nine-eleven. Exactly why this event has acquired this phrase is lost in the high emotions and intensive media coverage that surround the event. However, it is true that the 11th day of September was proclaimed 9-1-1 Emergency Telephone Number Day in 1987 as a promotional exercise. The promotional effort was often led by firefighters and the police. When the twin towers of the World Trade Center collapsed, many emergency service workers were killed and most of the responding vehicles were destroyed by the falling debris. Such prominent and world-wide coverage was given by the news media that somewhere the association between the date and the number it promotes was made. The phrase has became a short but meaningful acronym for the events that occurred on that day. It is seen by some as a fitting and appropriate memorial to those who lost their lives.